1. Field of the Invention
The present invention relates generally to shaft seals and, more particularly, is concerned with a thermally stabilized hydrostatic sealing assembly for a reactor coolant pump used in a nuclear power plant.
2. Description of the Prior Art
In pressurized water nuclear power plants, a reactor coolant system is used to transport heat from the reactor core to steam generators for the production of steam. The steam is then used to drive a turbine generator. The reactor coolant system includes a plurality of separate cooling loops, each connected to the reactor core and containing a steam generator and a reactor coolant pump.
The reactor coolant pump typically is a vertical, single stage, centrifugal pump designed to move large volumes of reactor coolant at high temperatures and pressures, for example 550 degrees F and 2500 psi. The pump basically includes three general sections from bottom to top--hydraulic, shaft seal and motor sections. The lower hydraulic section includes an impeller mounted on the lower end of a pump shaft which is operable within the pump casing to pump reactor coolant about the respective loop. The upper motor section includes a motor which is coupled to drive the pump shaft. The middle shaft seal section includes three tandem sealing assemblies--lower primary, middle secondary and upper tertiary sealing assemblies. The sealing assemblies are located concentric to, and near the top end of, the pump shaft. Their combined purpose is to mechanically contain the high positive pressure coolant of the reactor coolant system from leakage along the pump shaft to the containment atmosphere during normal operating condition. Representative examples of pump shaft sealing assemblies known in the prior art are the ones disclosed in U.S. Pat. Nos. to MacCrum (3,522,948), Singleton (3,529,838), Villasor (3,632,117), Andrews et al (3,720,222) and Boes (4,275,891) and in the first three patent applications cross-referenced above, all of which are assigned to the same assignee as the present invention.
The lower primary sealing assembly is the main seal of the pump. It is typically a hydrostatic, radially tapered "film-riding", controlled-leakage seal whose primary components are an annular runner which rotates with the pump shaft and a non-rotating annular seal ring which is attached to the housing of the lower seal assembly The annular runner typically includes an annular runner faceplate member mounted by a hydrostatic clamp ring member to an annular runner base or support member which, in turn, is keyed to the pump shaft for rotation therewith The annular seal ring typically includes an annular ring faceplate member mounted by a hydrostatic clamp ring member to an annular ring base or support member which, in turn, is keyed to the seal housing so as to prevent rotational movement of the seal ring relative to the seal housing but allow translatory movement of the seal ring along the pump shaft toward and away from the runner which rotates with the pump shaft
Historically, the pump shaft seals constitute the main problem area for the reactor coolant pumps and significantly contribute to the utilization factor in nuclear power plants The seals must be capable of breaking down the high system pressure (about 2500 psi) safely Whereas the tandem arrangement of three seals is used to break down the pressure, the lower main seal absorbs most of the pressure drop (approximately 2250 psi). Being a hydrostatic "film-riding" seal, the lower seal is designed to "lift off" (separate) at low system pressures by a hydrostatic pressure force present in the gap between the stationary seal ring and the rotating runner. A closing or seating force, which must balance the lifting force, is produced by the system pressure acting on the surfaces opposite the film surfaces of the seal ring and runner.
One of the potential problems associated with the lower seal stems from its response to a change of pump inlet water temperature which causes an axial temperature gradient to develop across the rotating runner and stationary seal ring faceplate members. The occurrence of increasing water temperature which creates an axially increasing temperature gradient in the faceplate members is of little concern in view that the flow path between the faceplate members becomes more divergent, resulting in a temporary condition where the film thickness, stiffness and leakrate are greater than normal. Thus, there is no risk of a wiping action taking place between the faceplate members which could produce seal failure. On the other hand, the occurrence of axially decreasing water temperature is of great concern in view that the flow path between the faceplate members becomes more convergent, resulting in a temporary condition where the film thickness, stiffness and leakrate are less than normal. The terminal response to this type of gradient would be a seal ring and runner with nearly parallel seal faces which would wipe one another and possibly fail.
Consequently, a need exists for an approach to hydrostatic sealing assembly construction which will avoid the potential deleterious effects which can arise from development of an axially decreasing water temperature gradient across the seal ring and runner faceplate members.